4th beta for initial release

• enable 'sos' as output format for IIR filters when scipy > 0.16 is installed
• new routines fil_convert and fil_save allowing to calculate filters in more than one output format (e.g. 'ba' and 'zpk') without needing to convert between both formats - important for ma - filter
• start making API for filter modules more transparent
• fix a bug where frequency range sometimes was wrong after zooming out full
• make calculation of phase more robust against inf or nan in H - unwrapped phase could not be calculated at all in this case
• show 'a' column in input_coeffs when manually designed filter (e.g. from input_pz) is IIR